The electrodes to which the chips are welded are formed of a metal with nickel as a main component, for example. Generally, the thermal expansion coefficient of the chips is smaller than the thermal expansion coefficient of the electrodes to which the chips are welded. Thus, at high temperature, the difference in thermal stress between the chips and the electrodes becomes relatively large. As a result, oxide scales are rapidly formed between the chips and the electrodes as a thermal cycle is repeated, possibly resulting in the peeling (detachment) of the chips from the electrodes in an early period.
In order to prevent the peeling (detachment) of the chip, the chip may be welded to the electrode very strongly so that the formation of oxide scales between the chip and the electrode can be suppressed. However, in this case, a part of the chip on the electrode side is pulled by thermal expansion of the electrode and is thus thermally expanded (deformed) more. Thus, the difference in thermal expansion between the part of the chip on the electrode side and a part of the chip on the opposite side from the electrode is increased. As a result, the part of the chip on the opposite side from the electrode may be deformed (such as warped), or breakage may be caused in the part.
The present invention is made in view of the above problems, and a purpose of the present invention is to improve the weldability of a chip to an electrode in a spark plug in which the thermal expansion coefficient of the chip is smaller than the thermal expansion coefficient of the electrode to which the chip is welded, so that deformation or breakage of the chip can be effectively prevented while peeling (detachment) of the chip from the electrode can be more reliably prevented.